BAGHDAD — U.S. and Iraqi troops yesterday stepped up patrols in Baghdad, establishing new checkpoints and randomly searching cars to stop the violence in the capital.

Prime Minister Nouri al-Maliki said the sweep, part of a new security crackdown code-named Operation Imposing Law, would target those “who want to continue with rebellion.”

There were conflicting reports, meanwhile, about the whereabouts of Muqtada al-Sadr, whose militia has been blamed for some of the worst sectarian killings in the past year, after a U.S. official said the radical Shi’ite cleric had fled to Iran ahead of the security operation.

Maj. Gen. William Caldwell, the chief U.S. military spokesman, insisted that Sheik al-Sadr had left the country, although he declined to comment on the reasons or give other details.

“We will acknowledge that he is not in the country and all indications are in fact that he is in Iran,” Gen. Caldwell told reporters in Baghdad.

But several of the cleric’s supporters denied the reports, with one official saying the cleric had met with government officials late Tuesday in the Shi’ite holy city of Najaf, where he has his headquarters.

Lawmaker Nassar al-Rubaie, the head of Sadrist bloc in parliament, also insisted that Sheik al-Sadr had not left the country.

“The news is not accurate because Muqtada al-Sadr is still in Iraq and he did not visit any country,” Mr. al-Rubaie said.

Also yesterday, CNN reported that a Shi’ite militant group had issued a video of Iraqi-born U.S. Army translator Ahmed Qusai al-Taayie, a 41-year-old reserve soldier from Ann Arbor, Mich., who was abducted by gunmen on Oct. 23.

Gen. Caldwell said officials were aware of the video and were analyzing a copy of it.

A U.S. soldier died yesterday after coming under small arms fire a day earlier from insurgents while on patrol north of Baghdad, the military said. Separately, another soldier died Tuesday in a non-combat-related incident that is under investigation, it said.

The U.S. military announced last week that the security clampdown in Baghdad had begun, but Iraqis had seen little evidence of that before yesterday.

As the new checkpoints were set up in the city of 6 million people, huge traffic jams developed and cars were forced to zigzag through soldiers’ positions and barricades.

Mr. al-Maliki discussed the security plan during a meeting with officials in the Shi’ite holy city of Karbala, 50 miles south of Baghdad.

“We have started today the new security plan. And we warn everyone who runs against it: Now we are ready to impose law on all those who want to continue with rebellion,” he said. “Baghdad operations started today under code name ‘Imposing Law.’ ”

He gave the pep talk a day after the Iraqi commander of the Baghdad security crackdown, Lt. Gen. Abboud Qanbar, announced that Iraq will close its borders with Syria and Iran and ordered the return of unlawfully seized homes as part of the drive to end the violence that has threatened to divide the capital along sectarian lines.

Despite the stepped-up security, a parked car bomb exploded in a predominantly Shi’ite district elsewhere in central Baghdad, killing four civilians and wounding 10, police said.

In the western city of Ramadi, a suicide car bomber struck a police station, killing at least eight policemen and wounding seven, police said. A suicide car bomber also targeted an Iraqi army patrol in the northern city of Mosul, killing one soldier and four civilians, police said.